1. Field of the Invention
The present application relates to a method of fabricating a structure with field-effect transistors each comprising a source electrode, a drain electrode, a channel extending between the source and drain electrodes and at least one gate electrode associated with the channel for controlling the conductance of the channel, wherein the channel comprises one or more semiconducting single-wall carbon nanotubes.
2. Prior Art
Single-wall carbon nanotubes (SWCNTs) are prospective components of nanoscale and molecular electronic devices. For instance, field-effect transistors (FETs) have been successfully fabricated from single semiconducting SWCNTs. The overall performance of SWCNT-FETs has been reported to be superior to that of state-of-the-art silicon-based MOSFETs, as reflected in the values of the drive current and transconductance being higher by a factor of four. Moreover, devices implementing CVD-grown SWCNTs have shown extremely high hole mobilities of up to 9000 cm2/Vs. With palladium as the electrode material, Schottky-barrier-free ballistic FETs have recently been realized, which exhibit high drive currents, good transconductance and switching ratios of 106.
In contrast to the remarkable achievements made with devices based upon individual SWCNTs, the use of nanotubes in large-scale electronics still faces several limitations which arise from the fact that all fabrication procedures developed to date yield mixtures of metallic and semiconducting tubes. In addition, the SWCNTs produced by bulk methods such as the arc discharge or HiPCO process (high pressure pyrolysis of carbon monoxide) are entangled in bundles. As a consequence, the electrical transport through such SWCNT ensembles is usually dominated by metallic pathways, and therefore only weak electric field effects on the conductance are attainable. For the goal of fabricating nanotube FETs two strategies have heretofore been followed to overcome this problem.
First, various solution-based techniques have been devised to separate metallic nanotubes from semiconducting ones, i.e. before they are adsorbed onto the substrate. These techniques rely upon differences in the physissorption properties, the capability to undergo charge transfer reactions or the different dielectric constants of the two types of SWCNTs:
For example, dispersing a SWCNT soot with octadecylamine (ODA) in tetrahydrofuran results in a physissorbed coating of ODA onto the sidewalls of the semiconducting SWCNTs due to the higher affinity of the latter to ODA. The metallic SWCNTs can then be separated by precipitation.
Similarly, the SWCNTs can be dispersed with single-stranded DNA yielding SWCNTs wrapped with DNA molecules. Anion exchange chromatography can then be used to extract an enriched fraction of metallic tubes utilizing the difference in surface charge between the DNA-metallic and the DNA-semiconducting SWCNTs.
Still another method relies on the increased capability of charge-transfer complex formation by bromine with the metallic nanotubes, resulting in the ability to separate one type from the other just by using centrifugation. Recently, it has been demonstrated that the varying dielectric constants of the metallic and semiconducting tubes can be utilized in separating the metallic nanotubes alone from a suspension through alternating current dielectrophoresis.
However, in none of these techniques can a 100% separation of the metallic nanotubes from the semiconducting ones be achieved, which is mainly due to the fact that the methods used are not selective exclusively to either the metallic or the semiconducting tubes.
The other approach utilizes the fact that the semiconducting SWCNTs can be turned to the insulating (OFF) state via application of a gate voltage. By ramping the drain-source voltage to sufficiently high potentials under the presence of oxygen, it is possible to burn off the metallic nanotubes. This destructive approach which has until now successfully been applied to individual SWCNT bundles has the disadvantage that adjacent semiconducting tubes may be affected in a negative way. Specifically, high current densities in the metallic nanotubes can heat up adjacent semiconducting tubes in the bundle thereby inducing defects in the semiconducting SWCNTs which can deteriorate device parameters such as charge carrier mobility.